Intrinsic Constraints on Asymmetric Turbulent Transport of Scalars Within the Constant Flux Layer of the Lower Atmosphere

A widely used assumption in boundary layer meteorology is the z independence of turbulent scalar fluxes F s throughout the atmospheric surface layer, where z is the distance from the boundary. This assumption is necessary for the usage of Monin‐Obukhov Similarity Theory and for the interpretation of eddy covariance measurements of F s when using them to represent emissions or uptake from the surface. It is demonstrated here that the constant flux assumption offers intrinsic constraints on the third‐order turbulent transport of F s in the unstable atmospheric surface layer. When enforcing z independence of F s on multilevel F s measurements collected above different surface cover types, it is shown that increasing instability leads to a novel and universal description of (i) the imbalance between ejecting and sweeping eddy contributions to F s and (ii) the ratio formed by a dimensionless turbulent transport of F s and a dimensionless turbulent transport of scalar variance. When combined with structural models for the turbulent transport of F s , these two findings offer a new perspective on “closing” triple moments beyond conventional gradient diffusion schemes. A practical outcome is a diagnostic of the constant flux assumption from single‐level F s measurements.